Photovoltaic devices or solar cells have been recognized as one of the leading candidates of renewable energy sources capable of supplying large scale clean energy that the world needs. The key metric in solar cell performance is power conversion efficiency that can be measured from the J-V (current-voltage) characteristics under standard simulated solar radiation using a solar simulator system.
As part of an effort to push the power conversion efficiency in solar cells even higher, there is a need to perform a more comprehensive electrical characterization of the solar cell beyond basic “efficiency” measurements. Specifically, advanced characterization steps are needed, such as Dark-JV, Jsc-Voc (short circuit current—open circuit voltage) and Rseries (series resistance) Extraction (RsX), where one could obtain various parameters that reveal some information about junction characteristics, parasitic (shunt and series) resistance and pseudo J-V performance. This information is very valuable for solar cell research and development as this information provides deeper insight about which aspects of the solar cell still need improvement.
The Jsc-Voc and RsX measurements require a variable light intensity. Intuitively, varying light intensity could be achieved with a solar simulator by controlling a power level of the solar simulator, changing the distance between the solar cell and the solar simulator, or inserting neutral density filters with different attenuation factors. But these methods cannot be performed easily and effectively. Namely, constantly switching the solar simulator power level in a large dynamic range with a high rate will be very detrimental for the stability and lifetime of the lamp (typically a xenon lamp). Varying the distance would require some sort of mechanical assembly and associated controls to move the lamp up and down and thus would be impractical. Further, the resulting range of light intensities would be very limited and light uniformity would vary a lot. Inserting various neutral density filters is also unrealistic because too many filters will be needed, the process is difficult to automate and measurements cannot be performed at a reasonable speed.
Recently, most of the Jsc-Voc measurement works are based on a flash lamp method. See, for example, U.S. Pat. No. 7,309,850 issued to R. A. Sinton et al., entitled “Measurement of current-voltage characteristic curves of solar cells and solar modules” and R. A. Sinton et al., “A quasi-steady-state open-circuit voltage method for solar cell characterization,” 16th European Photovoltaic Solar Energy Conference (May 1-5, 2000), Glasgow, UK, the contents of each of which are incorporated by reference herein. Using the flash lamp method, measurements can be performed rapidly by employing high speed electronics. This method, however, suffers several drawbacks. It requires a separate system from the standard solar simulator system, the measurement of Jsc is rather indirect since it actually measures the light intensity and Jsc is assumed to be proportional with this intensity, and the light source is not guaranteed to have the same solar spectrum throughout a large range of intensities. On top of these drawbacks, additional light source setup and requirements or high speed electronics raise the cost of such a system, thus limiting its popularity.
Therefore, improved variable light intensity measurement techniques for advanced solar cell characterization, such as Jsc-Voc and RsX measurements, would be desirable.